The present invention relates to a waveguide-type optical device, such as a modulator or others, and an optical switch for changing over optical signals used in communication systems.
Because light enables high-speed data transmission, the optical communication is dominant in long-distance transmission, such as nucleus communication systems. Recently, the transmission band of the optical communication has been on increase. Coupled with the development of WDM (wavelength division multiplex) technology, this makes the optical communication increasingly speedy and allows the optical communication to have larger capacities. In the optical communication system, in order to widen a WDM network from one to one interconnection to an interconnection among a plurality of points, switches are necessary for changing over optical signals to the plurality of points.
Conventionally, as a switch for changing over optical signals, one of the type that an optical signal is converted temporarily to an electrical signal to be switched in the electric signal, and the electrical signal is converted again to the optical signal has been dominantly used. However, for above 10 Gbps of the data transfer rate, it is difficult to form devices using electrical switches. Then, in place of the electric switches, optical switches which change over paths of optical signals in light has been developed. Such optical switches make the light/electricity conversion unnecessary to thereby realize switches which do not depend on velocities (frequencies) of optical signals.
As a conventional optical switch, one using mechanical micro mirrors is known. To realize optical switches of higher integration, higher velocities and lower losses, optical switches using refractive index changes due to electrooptical effect of ferroelectrics have been developed. The latter optical switches are very prospective in terms of forming WDM networks. The electrooptical effect is a phenomenon that a refractive index of a material is changed by an applied electric field.
A typical electooptic material used in the optical switches using the electrooptical effect is lithium niobate (LiNbO3). PZT (PbZrxTi1−xO3) , PLZT (PbxLa1−x(ZryTi1−y)1−x/4O3), whose electrooptic constants are large, are also prospective.
As a structure of the optical switches using the electrooptical effect is know one that prism-shaped electrodes are disposed on the upper surface and the back surface of a slab optical waveguide comprising clad layers formed on the upper surface and the lower surface of a slab core layer of an electrooptic material. The slab optical waveguide is an optical waveguide comprising a slab-shaped core layer and no transverse waveguide structure.
When a voltage is applied between the electrodes disposed on the upper and the lower surfaces of the slab optical waveguide, a refractive index of the electrooptic material between the electrodes is changed by the electrooptical effect. Then, prism effect based on a refractive index difference between the region to which the voltage has been applied and region to which no voltage has been applied is generated, whereby a propagation direction of an optical signal can be deflected. Because a deflection angle of an optical signal is changed by a voltage applied to the electrodes, the voltage is controlled to thereby switch the optical signal to a prescribed output channel.
For the optical switches using the electrooptical effect to exert the electrooptical effect, crystal structure of the electrooptic material forming the core layer is very important, and the electrooptic material has been single crystal film, such as PZT and PLZT formed by sol-gel, CVD or others. However, it is very difficult to make thick films of these electrooptic materials, retaining their crystallinity. For practical deflection characteristics, it is necessary to make films of these electrooptic materials as thin as some μm's.
On the other hand, the input and outputs of the optical switches are usually optical fibers of single mode of a 1.3 μm- or a 1,55 μm-wavelength and about 10 μm-mode field diameters. Accordingly, when optical fibers, or optical waveguides having the mode field diameters matched with optical fibers are connected to some μm-thick single crystal film, as shown in FIG. 1, large connection losses are generated because of large mismatch of the mode fields.
Technologies for the interconnection between mode diameters having a large difference therebetween are used in connecting, for example, semiconductor lasers and optical fibers. In the Laid-Open Japanese Patent Publication No. Hei 06-27355 (1994), for example, a tapered optical waveguide is used to thereby reduce connection losses. In the Laid-Open Japanese Patent Publication No. Hei 11-64653 (1999), it is proposed to a wedge-shape optical waveguide to thereby vertically change a mode field. These technologies are applicable only to channel optical waveguides and are not applicable to slab optical waveguides, in which light is not transversely confined.